1. Field of the invention:
The present invention relates to a laser beam projecting unit and a laser writing apparatus, which can be used in laser printers, laser copying machines, laser beam machines, laser printing machines, laser plate-making machines and the like. More particularly, the present invention relates to a laser beam projecting unit capable of projecting a stable laser beam of an intensity corresponding to the level of halftone, and to a laser writing apparatus capable of yielding excellent reproduction of the halftone gradations of a stored image.
2. Description of the prior art:
Conventional methods for recording a halftone image in a laser writing apparatus or the like include a first method wherein the light output of a laser beam generating means such as a semiconductor laser is varied to give a gradation of halftone to each pixel formed on a recording medium, and a second method wherein each pixel formed on a recording medium is segmented into a plurality of sub-pixels, and while the light output of a laser beam generating means is stabilized at a fixed level, the laser beam is projected only onto an appropriate number of sub-pixels selected from the plurality of sub-pixels according to the halftone of each pixel, thereby attaining the recording of a halftone image.
However, the first method requires precise control to vary the light output of the laser beam generating means to a plurality of levels and yet to stabilize it at each level, which is extremely difficult. On the other hand, the second method has the problem that an appropriate number of sub-pixels must be selected from the plurality of sub-pixels to project the laser beam thereon so that the halftone of each pixel can be reproduced, which requires a complicated process.